/Engine/Source/Runtime/Core/Public/Misc/Fork.h
Returns true if the server process was launched with the intention to fork. This could be a process on a fork-supported platform that will launch real child processes. (-WaitAndFork is set) Or it could be a process that will simulate forking by tranforming itself into a child process via fake forking (-FakeForking is set)
static bool IsForkRequested()